As alkaline storage batteries, a nickel-cadmium storage battery, nickel-metal hydride storage battery, nickel zinc battery and the like are widely known. Each of these alkaline storage batteries comprises a positive electrode comprising nickel hydroxide as an active material and a negative electrode comprising cadmium, hydrogen storage alloy or zinc, a separator and an electrolyte. Each of the separator and electrolyte is interposed between the positive electrode and the negative electrode. A non-woven fabric sheet comprising polyolefin is used for the separator, and a potassium hydroxide aqueous solution or the like is used for the electrolyte (Power Sources 12, Research and Development in Non-mechanical Electrical Power Sources, 1988, p. 393-410).
The alkaline storage batteries share a common problem that they have higher self-discharge than non-aqueous electrolyte secondary batteries. Particularly, in the case of the nickel-metal hydride storage battery, corrosion of the hydrogen storage alloy in the negative electrode causes the constituent elements of the alloy to dissolve into the electrolyte. The dissolved elements are able to freely migrate to the separator, the positive electrode and the like, thereby accelerating the self-discharge of the battery.
A method for improving the self-discharge characteristics of the nickel-metal hydride storage battery is disclosed in Japanese Unexamined Patent Publication No. Hei 1-57568. The publication proposes the use of a non-woven fabric sheet comprising sulfonated polyolefin for a separator.
The incorporation of a water absorbent polymer into the battery is proposed in Japanese Unexamined Patent Publication No. Hei 5-258767. This publication, however, discloses hardly any details of the water absorbent polymer.
In addition, the use of an electrolyte comprising a water absorbent polymer is disclosed in U.S. Pat. No. 5,541,019.